(1) Field of the Invention
This invention relates in general to an electrophotographic element, and in particular to a novel electrophotographic element which comprises a photosensitive layer containing as an effective ingredient a disazo pigment having the general formula (I): ##STR2## (wherein Ar.sup.1 and Ar.sup.2 can each represent a phenylene group. However, when Ar.sup.1 is 1,4-phenylene group Ar.sup.2 is 1,2- or 1,3-phenylene group; when Ar.sup.1 is 1,3-phenylene group Ar.sup.2 may be any one of 1,2-, 1,3- and 1,4-phenylene groups; and when Ar.sup.1 is 1,2-phenylene group Ar.sup.2 is 1,3-phenylene group. X can represent aromatic rings such as benzene ring, naphthalene ring and the like, hetero rings such as indole ring, carbazole ring, benzofuran ring and the like or their substitution products; Ar.sup.3 can represent aromatic rings such as benzene ring, naphthalene ring and the like, hetero rings such as dibenzofuran ring and the like or their substitution products; and R can represent a hydrogen atom, a lower alkyl group, phenyl group or its substitution product.)
(2) Description of the Prior Art
As light-sensitive materials which have long been utilized in electrophotographic elements, there are widely known inorganic light-sensitive materials such as amorphous selenium, selenium alloys, calcium sulfide, zinc oxide, etc., poly-N-vinyl carbazole and its derivatives.
It is known that among them, the amorphous selenium and selenium alloys are universally put to practical use because of the fact that they possess exceedingly superior characteristics indispensable for electrophotographic elements. However, the amorphous selenium is observed to be defective in that its light sensitive wavelength region is limited to the blue region and is scarcely sensitive to the red region.
Various kinds of methods have hitherto been proposed in order to widen the sensitivity to the long wave regions. However, the fact is that the sensitivity of the amorphous selenium to the long wave light is not improved to such a degree as intended by those methods because the selection of the light sensitive wavelength region is subjected to various limitations. In the case of utilizing zinc oxide or cadmium sulfide as light-sensitive material, it is needed to add various kinds of sensitizers in order that said zinc oxide or calcium sulfide may serve for practical purposes, because the light sensitive wavelength region of these materials is also limited.
While the poly-N-vinyl carbazole, universally known as organic photoconductive material, is abundant in superior abilities such as transparency, film forming ability, flexibility, hole transportability and the like, it is observed to be defective in that it is itself scarcely sensitive to 400 to 700 nm visible light wave length.
As a photosensitive element which has successfully overcome the above mentioned deficiencies, Japanese Patent Publication No. 10496/1975 discloses one utilizing therein a charge transfer complex consisting of poly-N-vinyl carbazole and 2,4,7-trinitrofluorenone.
In Japanese Patent Publication Nos. 5349/1970, 3168/1974, 14914/1975, 10982/1976, etc. are further discussed photosensitive elements of the type which comprises, in successive layers; a charge carrier generating layer formed of amorphous selenium or selenium alloy and a charge transfer layer, each layer being designed to play a part allotted thereto.
Still further, the following different photosensitive elements of the type which comprises, in successive layers, a charge carrier generating layer including a different kind of pigment and a charge transfer layer, have hitherto been developed. U.S. Pat. No. 3,837,851 discloses a photosensitive element which comprises a charge carrier generating layer and a charge transfer layer including at least one tri-arylpyrazoline. In U.S. Pat. No. 3,850,630 is disclosed a photosensitive element which comprises a transparent charge transfer layer and a charge carrier generating layer including an indigo dye. U.S. Pat. No. 3,871,882 discloses a photosensitive element which comprises a charge carrier generating layer including a perylene pigment derivative and a charge transfer layer including a condensate of 3-bromopylene and formaldehyde. Japanese Laid-open Patent Application No. 133445/1978 discloses a photosensitive element which comprises a transparent charge transfer layer and a charge carrier generating layer including a disazo pigment having astilbene skeleton. Some of the above mentioned photosensitive elements have already been commercially available. However, the fact is that there have not been developed yet photosensitive elements capable of satisfying various proposed properties in a satisfactory degree.